The northbound carriageway had been closed between junctions 31 (A59 Preston New Road, Samlesbury) and 31a (Bluebell Way) after an incident at around 5am.
All lanes reopened by 7am and traffic is now moving, but there is still some congestions around the M61 Interchange at junction 30.
We're also seeing some delays through Preston and Fulwood along the New Hall Lane, Blackpool Road, Garstang Road and Eastway, after traffic was diverted off the motorway and through the city.
Some drivers said they didn't move for nearly an hour after police brought traffic to a halt to deal with the incident.
The force has not confirmed what the incident was at this stage, but drivers reported that overhead matrix signs warned of an "oncoming vehicle".
Following the conclusion of the incident, the force said: "We have now reopened the M6 Northbound between J31A (Bluebell Way) and J32 (Broughton) following the conclusion of a police incident.
"Many thanks for your patience."
The M6 diversion route before lanes reopened this morning was as follows:
- Exit M6 J31, Tickle Trout.
- Take the first roundabout exit, Preston A59.
- At the next roundabout take the second exit, Blackpool (A583).
- At the A6 junction turn right, (M6) (M55) Follow the A6 north to M55 J1, Broughton.
- Take the third roundabout exit and follow the M55 to M6 J32, Blackpool. Rejoin M6 northbound.
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